Most antennas are passive devices which either radiate or receive electromagnetic radiation. A passive antenna structure can either transmit or receive, and an antenna's transmitting properties can be derived from its receiving characteristic or vice versa. The antenna is connected to a transmission line which carries an electrical signal that is transformed into electromagnetic radiation (in a transmitting antenna) or transformed from electromagnetic radiation (in a receiving antenna). An antenna design should be able to meet the desired criteria for gain, beamwidth, sidelobe level, polarization performance, and bandwidth requirements, while maintaining size/profile (including weight), cost of fabrication, and ease of fabrication at a minimum.
Tapered slot antennas are printed, travelling wave antenna structures which can provide a very wide bandwidth and are also relatively inexpensive to fabricate and integrate with a microwave transmission line. These antennas have a slot which is etched between metallization layers either on the surface of a dielectric substrate or in air. The slot tapers into a narrow slot line which is commonly fed by a microstrip line or other printed transmission line. The microstrip line is a strip conductor which is separated from a ground conductor by a dielectric substrate. However, a problem with the microstrip line is that it has a high transmission loss at high frequencies.
Phased arrays of tapered slot elements provide improved beam reconfiguration capability and improved beam pattern characteristics, particularly in terms of antenna gain. However, prior art feed techniques have limited the number of elements that can be combined into a tapered slot antenna array, because the increasing complexity of prior art feed structure results in increasing transmission losses. At the same time, if feed structures other than printed transmission lines are used, this results in an increase in the antenna array size, particularly thickness, and also in a band-limiting effect.
For instance, U.S. Pat. No. 5,036,335 to Jairam relates to a 45.degree. twist balun configuration for a microstrip line fed tapered slot antenna which improves the return loss of the antenna over a desired bandwidth. However, the feed structure disclosed by Jairam remains unsuitable for feeding a large array of antenna elements, for the reasons given above. Furthermore the structure disclosed by Jairam provides only a limited ability to optimize the return loss for different frequency bands.
As a result, there is a need for structures capable of feeding a tapered slot antenna, and particularly arrays of these antenna elements, which minimize transmission losses, provide a wide band transition that does not significantly curtail the wide band properties of the tapered slot element, is of small size, allow for easy and inexpensive fabrication and integration, and still enable desired performance requirements (including return loss, gain, beamwidth, sidelobe levels, and cross-polarization criteria) to be met.